SiGe, as a reliable and most efficient high-temperature thermoelectrics, has been utilized in special fields for many years, but there is no large-scale commercial application due to its high cost and low efficiency. Therefore, it is necessary to improve the dimensionless figure of merit ZT of a Si-based system, free of Ge which is expensive and rare earth, thereby becoming competitive in cost and efficiency for the commercial application. Since pure silicon possesses rather low ZT, for example 0.01 at room temperature, we have developed doped and nano-structured Si100P2.5 (GaP)1.5 bulk material and obtained ZT 0.47. In this work, a new approach to inducing random pores with four size distributions of 50 nm, 100 nm, 300 nm, and 1-2 μm is applied to the Si100P2.5 (GaP)1.5 bulk material, and ZT is improved by 32%. The increase of ZT can be attributed to the enhancement of the electrical conductivity and the Seebeck coefficient, and the reduction of the lattice thermal conductivity. The enhancement of electrical conductivity is ascribed to the doping effect of a small amount of Sb, while the increase of Seebeck coefficients stems mainly from the filter of low-energy carriers, and the reduction of lattice thermal conductivity arises mainly from phonons scattering. It is proved in this work that inducing random pores is an effective approach to improving the figure of merit of Si-based system.